Since some people move the wp-content folder out of the WP directory (specified here http://codex.wordpress.org/Editing_wp-config.php#Moving_wp-content) you need the first set of lines in securityscan.php to
$content = (defined('WP_CONTENT_DIR')) ? WP_CONTENT_DIR : ABSPATH . "wp-content";
require_once($content . "/plugins/wp-security-scan/support.php");
require_once($content . "/plugins/wp-security-scan/scanner.php");
require_once($content . "/plugins/wp-security-scan/password_tools.php");
require_once($content . "/plugins/wp-security-scan/database.php");
require_once($content . "/plugins/wp-security-scan/functions.php");
if(!class_exists("SimplePie")){
require_once($content . "/plugins/wp-security-scan/simplepie.inc");
}